1. Field of the Invention
The present invention generally relates to automatic gain control in a receiver, and more specifically to a programmable gain amplifier (PGA) that performs automatic gain control while minimizing transient voltages during tap changes.
2. Background Art
In electronic communications, electromagnetic signals carry information between two nodes over a connecting medium. Exemplary media include cable, optical fiber, public airways, etc. The signal strength at the receiving node varies depending on the distance between the nodes and changes in the condition of the medium. For example, the signal strength typically decreases with increasing distance between the two nodes. Furthermore, even if the distance is fixed, physical variations in the medium over time can affect signal strength. For example, in a cable system, different cables can have different attenuation constants. Also, increased moisture content in a cable line, or in the public airways can reduce signal strength at the receiver. Finally, variations in transmitter output power will also affect signal strength at the receiver.
An automatic gain control (AGC) circuit and a programmable gain amplifier (PGA) are often used at the receiver input to compensate for variations of received signal strength. More specifically, the AGC circuit adjusts the gain setting of the PGA to maintain the signal strength within a desired operating range. If the received signal strength is too high, then the AGC lowers the gain setting of the PGA. If the received signal strength is too low, then the AGC raises the gain setting of the PGA. When the AGC is changing the gain of the PGA, there is a possibility of introducing a glitch in the system. The glitch manifests itself as an unwanted transient voltage that can cause a voltage detection error if the transient voltage does not settle within specified time period, for example one clock cycle.
What is needed is PGA configuration that quickly settles any transient voltage caused by changing gain settings. Furthermore, the PGA configuration should have sufficient operating bandwidth.